FIG. 10(a) and FIG. 10(b) show a prior art of a DC—DC converter for stepping up or stepping down a direct current voltage input from a direct current power source such as a battery in an input and output noninverting state (the state wherein the polarity of the direct current input voltage is the same as that of the direct current output voltage) and supplying the direct current voltage to a load (See Japanese Patent No. Sho 58-40913). Voltage step-up is to output a direct current output voltage higher than a direct current input voltage, and voltage step-down is the reverse thereto. FIG. 10(a) is a circuit diagram of the above-mentioned DC—DC converter disclosed as the prior art, and FIG. 10(b) is a waveform diagram showing signals at each section thereof during the operation.
As shown in FIG. 10(a), this DC—DC converter is connected to a direct current input power source 31 for a voltage Ei and provided with a voltage step-down converter section consisting of a first switch 32, a first diode 33 and an inductor 34, a step-up converter section consisting of a second switch 35 and a second diode 36, having the inductor 34 in common, and an output capacitor 37. A voltage Eo of the output capacitor 37 is applied to a load 38 as a direct current output voltage.
As shown in FIG. 10(b), the first switch 32 and the second switch 35 are turned ON and OFF in the same switching cycle-T. The ratios of ON time of the first switch 32 and the second switch 35 per one switching cycle are referred to as a duty ratio δ 1 and a duty ratio δ2, respectively. As shown in the figure, the duty ratio δ1 is made larger than the duty ratio δ2: (δ1>δ).
When both the first switch 32 and the second switch 35 remain ON, the voltage Ei of the direct current input power source 31 is applied to the inductor 34. The time of application is the product of the duty ratio 2 by the switching cycle T: (δ2·T). At this time, a current flows from the direct current input power source 31 to the inductor 34, whereby magnetic energy is stored. Next, when the second switch 35 turns OFF, the second diode 36 becomes conductive, and the voltage that is a difference between the direct current input voltage Ei and the direct current output voltage Eo: (Ei−Eo) is applied to the inductor 34. The time of application is a difference between the product of the duty ratio δ1 by the switching cycle T, and the product of the duty ratio δ2 by the switching cycle T: (δ1·T−δ2·T). During the time of application, a current flows from the DC input power source 31 to the output capacitor 37 via the inductor 34. Further, when the first switch 32 turns OFF, the first diode 33 becomes conductive, and the direct current output voltage Eo is applied to the inductor 34 in the opposite direction. This time of application is a time (T−δ1·T), and a current flows from the inductor 34 to the output capacitor 37, whereby the stored magnetic energy is released.
As mentioned above, by repeating the operation of storage and release of the magnetic energy, electric power is supplied from the output capacitor 37 to the load 38. In a stable operation state wherein the storage and release of the magnetic energy of the inductor 34 balances, the sum of the products of the applied voltages and times of application is zero as represented by equation (1).Ei·δ2·T+(Ei−Eo)(δ1·T−δ2·T)−Eo(T−δ1·T)=0  (1)
By arranging this equation, a conversion characteristic equation represented by equation (2) is obtained.Eo/Ei=δ1/(1−δ2)  (2)
When the duty ratio δ2 is 0: (δ2=0), the ratio Eo/Ei of the direct current output voltage Eo to the direct current input voltage Ei becomes δ1: (Eo/Ei=δ1) and the converter operates as a voltage step-down converter. Further, when the duty ratio δ1 is 1: (δ1=1), the ratio Eo/Ei becomes 1/(1−δ2): (Eo/Ei=1/(1−δ2), and the converter operates as a voltage step-up converter. By controlling the duty ratios of the first and second switches 32, 35 respectively, the ratio of input and output voltages δ1/(1−δ2) can be set at 0 to infinity. In other words, the DC—DC converter operates as a voltage step-up and step-down converter theoretically capable of obtaining arbitrary direct current output voltage Eo from arbitrary direct current input voltage Ei.
The above-mentioned control of DC—DC converter can be carried out with, for example, a DC—DC converter having a control circuit 50 shown in FIG. 11(a) (See U.S. Pat. No. 4,395,675). For convenience of description, the circuit diagram shown in FIG. 11(a) is rewritten by applying the circuit described in FIG. 9 of the U.S. Pat. No. 4,395,675 to the DC—DC converter with the configuration as shown in FIG. 10(a). FIG. 11(b) shows operation waveforms of each section thereof. Operation of the DC—DC converter shown in FIG. 11(a) will be described below with reference to FIG. 11(b).
In FIG. 11(a), a reference voltage Vr is output from a reference voltage source 40 of the control circuit 50 and applied to an error amplifier 41. The error amplifier 41 compares the direct current output voltage Eo with the reference voltage Vr and outputs a first error voltage Ve1. An oscillation circuit 42 outputs an oscillation voltage Vt that oscillates at a predetermined cycle. An offset circuit 44 receives the first error voltage Ve1 as an input and adds a predetermined offset voltage to the first error voltage Ve1 to output a second error voltage Ve2.
FIG. 11(b) shows waveforms of the oscillation voltage Vt, two error voltages Ve1 and Ve2, and two driving signals Vg32 and Vg 35. A first comparator 43 compares the first error voltage Ve1 with the oscillation voltage Vt, and outputs the driving signal Vg 35 that becomes “H” during a period when the first error voltage Ve1 is larger than the oscillation voltage Vt: (Ve1>Vt) (“H” indicates “high” of logical level). It is assumed that when the driving signal Vg35 is “H”, the second switch 35 turns to ON state, and when it is “L”, the switch turns to OFF state (“L” indicates “low” of logical level). A second comparator 45 compares the second error voltage Ve2 with the oscillation voltage Vt, and outputs the driving signal Vg 32 that becomes “H” during a period when the second error voltage Ve2 is larger than the oscillation voltage Vt: (Ve2>Vt). It is assumed that when the driving signal Vg32 is “H”, the first switch 32 turns to ON state, and when it is “L”, the switch turns to OFF state.
In the case where the direct current input voltage Ei is sufficiently higher than the direct current output voltage Eo as a control target, the first error voltage Ve1 and the second error voltage Ve2 become lower in a stable state of the direct current output voltage Eo. During the period shown by A in FIG. 11(b), when the first error voltage Ve1 is lower than the oscillation voltage Vt at all times, the driving signal Vg35 becomes “L” at all times and the second switch 35 turns to OFF state at all times. On the other hand, the driving signal Vg32 that is set based on the comparison between the second error voltage Ve2 and the oscillation voltage Vt drives the first switch 32 to be turned ON and OFF. In other words, operation is made as a voltage step-down converter during the period A in FIG. 11(b).
In the case where the direct current input voltage Ei has a voltage in the vicinity of the direct current output voltage Eo as a control target, as in the period shown by B in FIG. 11(b), the waveforms of both the first error voltage Ve1 and the second error voltage Ve2 intersect the waveform of the oscillation voltage Vt. Therefore, the first switch 32 is driven to be turned ON and OFF by the driving signal Vg32, and the second switch is driven to be turned ON and OFF by the driving signal Vg35. In other words, the operation is made as a voltage step-up and step-down converter during the period B in FIG. 11(b).
Furthermore, in the case where the direct current input voltage Ei is lower than the direct current output voltage Eo as a control target, when the second error voltage Ve2 becomes higher than the oscillation voltage Vt at all times as in the period shown by C in FIG. 11(b), the driving signal Vg32 becomes “H” at all times and the first switch 32 turns to ON state at all times. On the other hand, the driving signal Vg35 that is set based on the comparison between the first error voltage Ve1 and the oscillation voltage Vt drives the second switch 35 to turn ON and OFF. In other words, the operation is made as a voltage step-up converter during the period C in FIG. 11(b).
The ON and OFF timing of the first switch 32 and the second switch 35 shown in FIG. 11(b) is different from the ON and OFF timing of the first switch 32 and the second switch 35 shown in FIG. 10(b). This difference depends on the difference between control circuits shown in FIG. 10 and FIG. 11 in constitution and function. Combinations of ON and OFF of the first switch 32 and the second switch 35 in the DC—DC converter are basically the following three types: both the first switch 32 and the second switch 35 are in ON state; the first switch 32 is in ON state and the second switch 35 is in OFF state; and both the first switch 32 and the second switch 35 are in OFF state. In the case where the first switch 32 is in the OFF state and the second switch is in the ON state, the inductor 34 is short-circuited, and having no relation with transmission of electric power between input and output, and therefore the state of operation should be avoided. On the condition that any combination of the above-mentioned three types of operation states never causes a current flowing to the inductor 34 to become zero, when the ratio of the ON time of the first switch 32 per one switching cycle is referred as to δ1 and the ratio of the ON time of the second switch 35 per one switching cycle is referred as to δ2, the relation of the following equation (3) is established between input and output voltages. This also applies to the timing of the ON and OFF operation of each switch shown by waveforms in FIG. 10(b) as well as the timing of ON and OFF of each switch shown by waveforms in FIG. 11(b).Eo/Ei=δ1/(1−δ2)  (3)
Other examples of method for controlling a DC—DC converter capable of carrying out voltage step-up and step-down are disclosed in U.S. Pat. No. 5,402,060 and U.S. Pat. No. 6,166,527. Both of these compare the oscillation voltage with the error voltage and add or subtract an offset voltage to or from the oscillation voltage or the error voltage so that a driving signal for driving the first switch and a driving signal for driving the second switch are formed.
The DC—DC converter of the above-mentioned U.S. Pat. No. 4,395,675 requires a plurality of error voltages Ve1 and Ve2, thereby to cause the problem of complicating the control circuit.
Further, during voltage step-up and step-down operation in which both the first switch 32 and the second switch 35 is turned ON and OFF, there causes the problem of increasing switching loss compared with during voltage step-down operation or voltage step-up operation. In order to narrow the region where the voltage step-up and step-down operation is carried out to solve the problem, it is necessary that the offset voltage to be added to the error voltage is made to a voltage in the vicinity of an amplitude of the oscillation voltage. However, when the offset voltage is made to the voltage in the vicinity of the amplitude of the oscillation voltage, fluctuation band of the error voltage becomes larger in order to ensure control range in step-down voltage operation and step-up voltage operation. For that reason, in the case of low power supply voltage of the control circuit, there has caused a problem of difficulties in design.